The present invention relates to a device for measuring the space distribution of the spectral emission of an object.
It applies in particular to objects such as, for example, projection screens, cathode ray tubes, lighting devices, flat display screens such as liquid crystal screens, plasma screens, electroluminescent screens and microtip screens, as well as reflecting surfaces.
Several techniques are already known for measuring the calorimetric specifications of the emission or reflection of light by various objects (for example those mentioned above) as a function of the angle of observation.
Reference can be made to the following documents on this subject:
[1] EP 0 286 529 A
[2] FR 2 729 220 A
[3] FR 2 749 388 A
In particular an electromechanical technique is known which consists of moving measuring equipment, such as a photometer, around an object which one wishes to measure. On this subject, reference can be made, for example, to document [1] and in particular to FIGS. 2a, 2b and 3 of this document [1].
This known technique presents many inconveniences.
In particular, the measurement is carried out by sampling. Only the positions chosen are measured and no information is known about the luminance in the intermediary positions or angles. No certitude exists about the luminance value apart from that at the measured points.
In addition, the measurements, of duration T0, are carried out in series, one after the other. If a large number of points N are to be measured in order to be able to obtain a maximum of information, the complete measurement of the object takes a time NxT0.
Another technique is known which uses a Fourier optic associated with an array sensor of the CCD type or other. On this subject, reference should be made, for example, to document [3] and in particular to FIG. 1 of this document [3].
This other known technique consists of obtaining, in one go, the space distribution of the light emitted or reflected on this matrix sensor by an object to be measured. The different points of the image correspond to the measurements obtained for the emission specifications of the object to be measured, at different angles.
The principal advantages of this other technique are the following:
The speed of measurement is higher. In fact the measurement of duration T1 does not depend on, or depends little on, the number of points measured.
All the information is available.
There is no risk that a detail of the angular distribution of luminance escapes being measured.
An integration (summation) of the overall values obtained gives with certainty a value for the luminous flux emitted by the object.
Nonetheless, a serious inconvenience has become apparent during usage of this other known technique. In fact, (and referring to documents [1] and [2]), the measurement of luminous flux is carried out without reference to the distribution by wavelength of the collected light. This can be restrictive
if one is trying to obtain spectral variations in function of the angle of observation,
in colorimetry where it is sometimes complicated and always imprecise to calculate the colorimetric co-ordinates without knowing the spectral distribution of the light, and
in reflectometry where it is indispensable to know the spectrum of the light source used to illuminate an object in order to evaluate the capacity of the object to reflect any particular wavelength.
The aim of the present invention is to remedy these inconveniences.
Its objective is a device which combines the advantages of the technique described in document [3], advantages which have been seen above, with the advantages of spectral measurement.
Certainly, one could envisage (as do documents [2] and [3]), placing a series of selective filters in the path of the luminous signal, making it possible to select a range of wavelengths.
Nonetheless, given that the spread of the visible spectrum is 340 nm (since this visible spectrum ranges from 380 nm to 720 nm) and that the resolution needed for photometric measurements is of the order of 4 nm, 85 successive filters (and 85 successive measurements) would be necessary. Such a technique would therefore lead to long measuring times.
Furthermore, the setting up of 85 filters in the path of a luminous signal is not simple and the cost of these 85 filters is not negligible, since the manufacture of filters with a 4 nm bandpass is especially delicate.
Nevertheless one could think of using a restricted number of filters, each filter with a 10 nm bandpass. But then there would be only 34 filters and the precision of the measuring device would be lowered.
The device which is the subject of the invention combines the advantages indicated above without presenting the inconveniences which have just been described concerning this utilisation of a series of selective filters.
To be precise, this present invention has the aim of a device for measuring the space distribution of the spectral emission of a measurement zone of an object, this device being characterised in that it comprises:
a first lens envisaged to form, in the Fourier plane of this first lens, a first image constituting the optical Fourier transform of the zone of measurement,
a first diaphragm
a second lens set between the first lens and the first diaphragm and co-operating with this first lens so that the opening of the first diaphragm shall be conjugated optically with the measuring zone by the first and second lenses and that the measurement zone, when it is observed through the first diaphragm, has an apparent surface approximately independent from the direction of observation, the first and second lenses having a common optical axis which constitutes the optical axis of the device,
means of selection of a rectilinear portion of the first image following a direction of selection,
means of light dispersion, envisaged to disperse the light corresponding to the selected portion of the first image,
a bidimensional image sensor, envisaged to receive the light thus dispersed and to provide signals representative of this dispersed light, and
means of treatment of these signals, able to determine the spectral response of the measurement zone for each point of the rectilinear portion of the first image.
Preferably, the first diaphragm has a circular opening.
Preferably also, the means of dispersion are able to disperse the light following a dispersion direction which is perpendicular to the direction of selection.
According to a preferred embodiment of the device which is the subject of the invention, the means of selection comprise a rectilinear slit which is formed through a material opaque to the light issued from the object.
According to a preferred embodiment of the device which is the subject of the invention, the direction of selection passes along the optical axis of the device.
In this case, the device which is the subject of the invention can also comprise means of rotation of the object around the optical axis of the device. This makes it possible to measure the specifications of the measurement zone for different values of the azimuth xcfx86.
Alternatively, the device which is the subject of the invention can also comprise means for displacing the means of selection, in such a way that the selection direction scans the first image, the means of selection thus selecting successive rectilinear portions of this first image.
In this case, according to a special embodiment of the invention, the means of selection comprise a rectilinear slit which is made through a material which is opaque to the light issuing from the object and which defines the direction of selection, this slit passing along the optical axis of the device, and the means of displacement are the means of rotation of the slit around this optical axis and the means of dispersion are able to disperse the light following a dispersion direction which is maintained perpendicular to the direction of selection and thus to the slit.
Also in this case, according to another particular embodiment, the means of selection comprise a rectilinear slit which is made through a material which is opaque to the light issuing from the object and which defines the direction of selection, this slit passing along the optical axis of the device, and the means of displacement are means of rotation of the slit around this optical axis and the device also comprises de-rotator means which are set between the slit and the means of dispersion and are envisaged to maintain constant the orientation of the dispersed light received by the sensor.
The device which is the subject of the invention can also comprise a luminous source and semi-reflecting means envisaged to reflect the light emitted by this luminous source towards the object so as to illuminate a zone of the latter containing the zone to be measured and to let pass the light issuing from this measurement zone thus illuminated and which is directed towards the first diaphragm.
In this case, according to a first particular embodiment, the semi-reflecting means are set between the second lens and the first diaphragm and the device comprises in addition:
a second diaphragm whose opening defines the illuminated zone, this opening of the second diaphragm being conjugated optically with the zone illuminated by the ensemble formed by the first and second lenses, and
a third lens,
the luminous source being set in a plane which is conjugated optically with the Fourier plane of the first lens by the ensemble formed by the second and third lenses.
In this case as well, according to a second particular embodiment, the semi-reflecting means are set between the first lens and the second lens and the device also comprises:
third dad fourth lenses, and
a second diaphragm whose opening defines the illuminated zone, this opening of the second diaphragm being conjugated optically with the illuminated zone by the ensemble formed by the first and fourth lenses,
the luminous source being set in a plane which is conjugated optically with the Fourier plane of the first lens by an ensemble formed by the third and fourth lenses.